The seven-time Tour de France winner has just returned to competition from a broken collarbone in his comeback season, after a four-year absence from the sport.

It will be the 37-year-old's first assault on the 22-day event and Armstrong is more concerned with helping Astana team-mate Levi Leipheimer than his own challenge for glory.

"I feel better than I expected to feel but I know I don't have the condition to win," said Armstrong, ahead of the start of Saturday's race.

"I could very well be past that point now and be putting myself up for a lot of suffering."

"We have a rider who can be a factor in the race - top five or top three. Levi is a complete rider. He can climb and time trial."

Since his return to the saddle, Armstrong has produced solid performances at the Tour of California - where he was seventh - and the lower ranked Tour de la Gila, where he was second.

Leipheimer however has been in superb form this season and has three overall victories to his name in 2009 in the Tour of California, the Tour de la Gila and the Vuelta a Castilla y León

 The 100th edition of the race will feature nine Brits including Mark Cavendish, who won two stages in 2008, David Millar and Bradley Wiggins.